Of the Box

Strong tides may have pushed ancient fish to evolve limbs

PORTLAND, OREGON—The evolution of land animals only happened once, some 400 million years ago. But what pressures pushed sea creatures to evolve limbs for walking? Scientists have proposed several theories, including fish that adapted to living in shallow, plant-choked streams prone to flooding and drought. Now, new research suggests that that strong ocean tides may have played a significant role, stranding animals in tidal pools and giving them an incentive to escape back to the sea. Using computer simulations of ancient Earth, the researchers find that regions of strong ocean tides correspond to locations where fossils have been found of large, bony fishes with limblike fins called sarcopterygians, researchers reported here today at the 2018 Ocean Sciences Meeting.

The hypothesis isn’t mainstream, says Jennifer Clack, a paleontologist at the University of Cambridge in the United Kingdom, who wasn’t involved in the research, “but I don’t think it’s entirely off the wall.”

Nearly a century ago, Alfred Romer, a paleontologist at the University of Chicago in Illinois, proposed that tidal pools might have helped spur the evolution of the earliest four-footed animals, known as tetrapods. In 2014, Steven Balbus, an astrophysicist at the University of Oxford in the United Kingdom, took the idea one step further. He calculated that 400 million years ago, when this evolutionary transition occurred, tides were stronger, because the moon was about 10% closer to Earth. Fish could become easily stranded in tide pools during stronger spring tides, he argued, which occur when Earth, the moon, and the sun are in alignment roughly every 2 weeks. Strandings would be particularly likely in places where the tides were naturally amplified by the local water depth or the shape of the coastline.

And stranded creatures would have been under evolutionary pressure to escape their watery confines, says Mattias Green, an ocean scientist at Bangor University in the United Kingdom. “After a few days in these pools, you become food or you run out of food,” he says. “The fish that had large limbs had an advantage because they could flip themselves back into the water.”

But providing the evidence to support this theory is tough. That’s because plate tectonics have shifted the position of Earth’s continents, and hundreds of millions of years of erosion and other changes have vastly changed the shape of coastlines. But Green, Balbus, and their collaborators have now tried, simulating Earth’s tides millions of years ago to look for coastlines with particularly strong daily tides and pronounced differences between the strong spring tides and their opposites—weaker neap tides. They used a model of Earth’s seafloor and landforms as they existed 400 million years ago, when two supercontinents called Gondwana and Laurussia were coming together, separated by a sea that resembled a wedge. “Having such a big wedge-shaped sea would lead to an enhanced tidal response,” Balbus says. By modeling different local topographies and recording the tides at each location, the researchers identified swaths of coastline along the wedge separating the supercontinents where fish could easily have been stranded.

The scientists showed that these ancient locations—shifted by plate tectonics to their present-day positions—overlap with many finds of “transitional” fossils of bony fish with limblike fins. For example, large fossil beds and impressions of footprints in modern-day Eastern Europe, Canada, and Ireland match the locations of ancient tidal stranding sites “almost disturbingly well,” Green says. Intriguingly, the simulations also suggest that transitional fossils could be found in other places, like Syria and Afghanistan. But excavating there is challenging because of current political instabilities, says Hannah Byrne, an oceanographer at Uppsala University in Sweden who led the modeling work.